The present invention relates to a magnetic flowmeter for measuring the flow rate of a fluid. More specifically, the invention relates to measuring fluid impedance and monitoring for an empty pipe condition in a magnetic flowmeter.
In an electromagnetic flowmeter, a pulsed magnetic flux is applied in a direction substantially perpendicular to the direction of flow of a fluid flowing through a measuring pipe. The flux induces a potential difference in the fluid varying as a function of the fluid flow rate, and the potential is sensed by two electrodes positioned in the pipe. In a flowmeter, it is useful to detect the impedance of fluid flowing through the measuring pipe. This information can be used to detect an empty pipe condition. One approach uses a DC current flowing between the electrodes and electrical ground and additional circuitry to detect the empty pipe condition, but this approach undesireably plates the electrodes so that they must be replaced. U.S. Pat. No. 4,969,363, issued Nov. 13, 1990 to Mochizuki, entitled Electromagnetic Flowmeter Capable of Simultaneous Measurement of Flow Rate and Conductivity of Fluid, discloses an empty pipe detector applying a current pulse to the electrodes to monitor fluid impedance only while the coil drive current is rising. As most flowmeters have multiple frequencies at which the coils are driven, so that the rise times of the coil drive current vary, the device must necessarily include timing circuitry which allows the device to be active for different rise times. Furthermore, some flowmeter installations have such a large cable capacitance that the fluid impedance measurement is hampered when the coil drive frequency is greater than about 35 Hz. As a result, Mochizuki's device is limited to low capacitance cabling applications, has additional timing circuitry and must be synchronized with the pulsed magnetic flux so that it cannot continuously monitor fluid conductivity.
Therefore, there is a need for a flowmeter with a continuously monitoring empty pipe detector with simple circuitry and which is independent of coil drive frequency but which does not polarize the electrodes, and operates with high capacitance cables.